Known in the art is a method for preparing vinylaromatic monomers such as styrene, vinyltoluenes or vinylxylenes by way of alkylation of benzene, toluene or xylene with ethylene into the ring, followed by dehydrogenation of the resulting alkylaromatic compounds. The alkylation is conducted at a temperature within the range of from 50.degree. to 120.degree. C., under a pressure of up to 10 atm in the presence of aluminum chloride or sulphuric acid. The dehydrogenation is performed at a temperature within the range of from 500.degree. to 700.degree. C. under atmospheric pressure on a heterogeneous catalyst. The yield of vinylaromatic compounds is as high as 80% by weight.
This prior art method has a disadvantage residing in that it makes possible to produce merely a mixture of isomers of vinyltoluene or vinylxylene which is impossible to separate for further economically efficient utilization thereof.
Also known in the art is a method for preparing styrene by way of heating monoalkylbenzene to a temperature within the range of from 550.degree. to 800.degree. C. in the presence of a sulphur-organic compound or a halogenalkyl.
As the monoalkylbenzenes use can be made of compounds of the formula: C.sub.6 H.sub.5 --CH.sub.2 --CH.sub.2 --R (i.e. propyl-, butyl-, pentylhexyl-, or heptylbenzene).
Thus, upon passing butylbenzene through a steel pipe at a temperature of 600.degree. C. in the presence of thiophenol, styrene is formed with a selectivity of 75.1 mol.% (58.2% by weight) compared to 55.6 mol.% in the absence of thiophenol.
Monoalkylbenzenes used for conversion to styrene are produced by alkylation of toluene into the methyl group by means of olefins.
This prior art method also has a disadvantage residing in a low selectivity of conversion of monoalkylbenzene to styrene (58.2 mol.%). Furthermore, in this prior art method a part of the alkyl radical is split-off in the form of light paraffin hydrocarbons (methane, ethane and the like) which cannot be efficiently employed.